Dual-mode antenna design for microwave heating and noninvasive thermometry of superficial tissue disease

Hyperthermia therapy of superficial skin disease has proven clinically useful, but current heating equipment is somewhat clumsy and technically inadequate for many patients. The present effort describes a dual-purpose, conformal microwave applicator that is fabricated from thin, flexible, multilayer printed circuit board (PCB) material to facilitate heating of surface areas overlaying contoured anatomy. Preliminary studies document the feasibility of combining Archimedean spiral microstrip antennas, located concentrically within the central region of square dual concentric conductor (DCC) annular slot antennas. The motivation is to achieve homogeneous tissue heating simultaneously with noninvasive thermometry by radiometric sensing of blackbody radiation from the target tissue under the applicator. Results demonstrate that the two antennas have complimentary regions of influence. The DCC ring antenna structure produces a peripherally enhanced power deposition pattern with peaks in the outer corners of the aperture and a broad minimum around 50% of maximum centrally. In contrast, the Archimedean spiral radiates (or receives) energy predominantly along the boresight axis of the spiral, thus confining the region of influence to tissue located within the central broad minimum of the DCC pattern. Analysis of the temperature-dependent radiometer signal (brightness temperature) showed linear correlation of radiometer output with test load temperature using either the spiral or DCC structure as the receive antenna. The radiometric performance of the broadband Archimedean antenna was superior compared to the DCC, providing improved temperature resolution (0.1 degree C-0.2 degree C) and signal sensitivity (0.3 degree C-0.8 degree C/degree C) at all four 500 MHz integration bandwidths tested within the frequency range from 1.2 to 3.0 GHz.     

Effect of complex bolus-tissue load configurations on SAR distributions from dual concentric conductor applicators. Specific absorption rate

Power deposition [specific absorption rate (SAR)] distributions from a two-element array configuration of 4-cm-square 915-MHz dual concentric conductor (DCC) microwave antennas were characterized theoretically for several clinically realistic complex bolus-tissue load models using the finite difference time domain (FDTD) numerical method. The purpose of this effort was to determine the perturbing effects on SAR of three often unavoidable heterogeneities in the bolus-tissue load. The three cases studied in this work consist of bone (two ribs spaced 1 cm apart) embedded 5-mm or 1-cm deep in muscle or layered fat-muscle tissue, small air bubbles trapped between the coupling bolus and tissue surface, and variable thickness water bolus layer due to sharply contoured anatomy. Results of the FDTD simulations demonstrate rather small effects on SAR distribution for both rib-sized bones > or = 5-mm deep in muscle and small air pockets < or = 1-mm thick. Larger air bubbles > 1-cm diameter by 3-mm depth showed a distinct concentration of SAR near the lateral sides of the air bubbles, and a blocking effect under the bubbles when located directly under the center of a DCC aperture where there is a higher normal E-field component. Variation from 2.5- to 7.5-mm bolus thickness under the two aperture array produced only minor perturbation of the uniformity and penetration of SAR, along with minor reduction in SAR under the thicker bolus which should be accommodated sufficiently by changes in applied power to the array elements.     

共享孔径交错稀疏阵列天线互耦误差建模与校正

共享孔径交错稀疏阵列天线是实现多功能阵列天线的有效途径。现有的参数化互耦消除方法都是针对均匀阵列天线展开的,其研究的互耦矩阵都是规则的方阵,对共享孔径交错稀疏阵列天线的互耦矩阵模型并不适用。在充分考虑共享孔径交错稀疏阵列天线中子阵内互耦的“稀疏”和“方位依赖”的特殊性后,通过将常规的互耦矩阵扩展表示为“非方”的“增广互耦矩阵”来对交错稀疏阵列天线子阵内和子阵间的耦合效应进行建模,并通过“增广互耦矩阵”的参数化估计最终实现了共享孔径交错稀疏阵列天线互耦误差的建模与校正。仿真结果证实了所提方法的有效性和可行性。     

Short helical antenna array fed from a waveguide

An array consisting of short helical antennas is fed from a single rectangular waveguide. Stubs are introduced near the array elements in order to make a matching condition between the array elements and the waveguide. On the basis of a transmission line theory, a coupling phase and a coupling factor are determined. The mechanical rotation is applied to each array element so that in-phase condition at the aperture may be formed. Excellent agreement between the calculated and experimental results is demonstrated in the arrays of five, seven, and nine helical antennas.     

The calculated and measured temperature distribution of a phasedinterstitial antenna array [invasive applicators]

The power deposition pattern of four antennas, positioned on the corners of a 2-cm square array with different driving phases, is computed under the assumption of negligible coupling between the antennas. The spatial SAR (specific absorption rate) distribution is calculated by modeling each interstitial applicator as an insulated, asymmetric dipole. For comparison with the heating patterns measured by a thermal video system, the calculated SAR distributions are converted into temperature patterns through an electric network simulation of the heating in artificial muscle tissue. At each nodal point of a grid in the thermal system, the absorbed microwave power (or SAR times density), thermal resistivity, heat capacitance, and temperature are simulated, respectively, as current source, electrical resistance, electrical capacitance and potential. Therefore, solving the equivalent electric network on a computerized simulation routine (SPICE) yields the temperature distribution. In both the axial and transverse planes, the resulting temperature distributions from the antenna array, with various driving phases, agree very well with the measured temperature patterns     

Current sheet applicators for clinical microwave hyperthermia

Small lightweight applicators, intended to be operated in array configurations are described. Their radiative fields are induced by RF currents in a conducting sheet embedded a few millimeters below the dielectric covered aperture surface. In arrays, these elements can be used where conformity to body curvatures is necessary. The clinical prototypes are tuned to 434 MHZ and have a bandwidth of almost 20 MHZ, which accommodates the tuning and coupling changes due to different body movement, and tissue heterogeneities. They are relatively insensitive to air bubbles in bolus and scar tissue. Their inherent linear polarization allows easy visualization of the superposition of electric field vectors of each element of an array and provides deeper penetration on curved surfaces due to electric field vector addition in the medium. In the case of a large breast tumor, depth of heating of over 4 cm was achieved along the central axis of a 2×2 coherent array. Experimental evaluation of these elements, leading to clinical implementation, is described along with a clinical example     

Design of a Compact Millimeter-Wave Microstrip Antenna with Wide Bandwidth and Broad Beamwidth

Design of a novel compact millimeter-wave microstrip antenna with wide bandwidth and broad beamwidth is presented. In the structure, multiple layers and a parasitic element as well as a coupling aperture are used to achieve wideband, while a conducting cylinder and the coupling aperture are used to obtain broad beamwidth. Finally, phased array with eight present elements has been designed as an example. Compared to traditional microstrip antennas and arrays, present antenna and array are of smaller dimension and broader beamwidth as well as wider bandwidth.     

Microwave hyperthermia induced by a phased interstitial antennaarray

An interstitial microwave antenna array for hyperthermia cancer treatment is investigated. The purpose is to generate both uniform and controlled nonuniform temperature distributions in biological tissue by modulating the phases of the signals applied to each antenna. The array has four antennas positioned on the corners of a 2 cm square. The distributions of absorbed power within the arrays are computed and then converted into temperature distributions through a heat conduction simulation. The temperature patterns over phantom muscle are presented in both the lateral plane (perpendicular to the antennas) and the axial plane (parallel to the antennas). It is found that by proper phase modulation of radiofrequency signals applied to each antenna, a uniform heating can be produced in the entire array volume     

A thermal monitoring sheet with low influence from adjacent waterbolus for tissue surface thermometry during clinical hyperthermia

This paper presents a complete thermal analysis of a novel conformal surface thermometer design with directional sensitivity for real-time temperature monitoring during hyperthermia treatments of large superficial cancer. The thermal monitoring sheet (TMS) discussed in this paper consists of a 2-D array of fiberoptic sensors embedded between two layers of flexible, low-loss, and thermally conductive printed circuit board (PCB) film. Heat transfer across all interfaces from the tissue surface through multiple layers of insulating dielectrics surrounding the small buried temperature sensor and into an adjacent temperature-regulated water coupling bolus was studied using 3-D thermal simulation software. Theoretical analyses were carried out to identify the most effective differential TMS probe configuration possible with commercially available flexible PCB materials and to compare their thermal responses with omnidirectional probes commonly used in clinical hyperthermia. A TMS sensor design that employs 0.0508-mm Kapton MTB and 0.2032-mm Kapton HN flexible polyimide films is proposed for tissue surface thermometry with low influence from the adjacent waterbolus. Comparison of the thermal simulations with clinical probes indicates the new differential TMS probe design to outperform in terms of both transient response and steady-state accuracy in selectively reading the tissue surface temperature, while decreasing the overall thermal barrier of the probe between the coupling waterbolus and tissue surface.      

Absorbed Power Distributions from Coherent Microwave Arrays for Localized Hyperthermia

Absurbed power distributions in a homogeneous muscle-like tissue due to a planar coherent array consisting of 16 small direct contact applicators at 434 and 915 MHz are calculated, assuming various relative phases and amplitudes are compared with that of a single aperture source at the same frequency with the same overall dimensions. The array applicator may offer improvement in field size or, when focused, a small improvement in penetration, but in practice the performance is very dependent upon bolus thickness. An additional advantage of the array applicator is the ability to modify absorbed power distributions during use by changing the amplitudes of individual applicators.     

Numerical simulation of annular phased arrays for anatomicallybased models using the FDTD method

Annular phase arrays (APAs) of aperture and dipole antennas used for hyperthermia are simulated in three dimensions by using the finite-difference time-domain (FDTD) method. A 17363 cell, 1.31 cm resolution, anatomically based model of the human torso surrounded by a bolus of deionized water is used for calculations of specific absorption rates (SARs). Test runs on the calculation of fields in the water-filled interaction space and with homogeneous circular- and elliptical-cylinder phantoms correlate well with the experimental data in the literature, lending support to the accuracy of the FDTD method for near-field exposure conditions. Results are given for APAs using different sizes of aperture and dipole antennas and for a subannular array to obtain higher SARs in the liver. It is concluded that, because of its flexibility, the proposed procedure may be useful for a variety of realistic radiofrequency applicators for hyperthermia and other biomedical applications     

共享孔径同心圆阵稀疏交错优化布阵方法

同心圆阵列天线具有波束对称、360°方位角扫描、抗干扰能力强等优点，被广泛应用于星载、机载、舰载雷达及飞机声呐等领域。在天线孔径有限的情况下，如何进一步提高同心圆阵的孔径利用率，通过孔径的空分复用，设计出子阵稀疏交错分布的多功能同心圆阵列天线，具有较大的研究价值。利用均匀同心圆阵列天线激励与方向图函数存在二维傅里叶-贝塞尔变换关系，基于二维三次插值和密度加权，提出了一种同心圆阵稀疏交错优化布阵的方法。该方法通过对均匀同心圆阵列天线方向图采样值的频谱能量进行分析，采用三次插值的方法，实现了同心圆天线阵列方向图函数到同心圆阵元激励能量的映射转换；基于密度加权的原理，对排序后归一化阵元激励的奇偶交错选取，使得稀疏交错子阵方向图频谱能量均分匹配，实现了同心圆阵的稀疏交错优化布阵设计。仿真结果表明，该方法得到的交错子阵天线具有峰值旁瓣电平低、主瓣宽度窄且方向图性能近似程度高的优点，有效解决了同心圆阵列天线稀疏交错优化布阵的设计难题，实现了两子阵交错的共享孔径多功能同心圆阵列天线设计。     

The RCS of a cylindrical array antenna coated with a dielectric layer

The scattering properties of dielectric coated waveguide aperture antennas mounted on circular cylinders are investigated. Both the single element antenna and the array case are treated. The array antenna consists of 4 /spl times/ 32 rectangular apertures placed in a rectangular grid on the surface of an infinitely long circular cylinder. The problem is formulated in terms of an integral equation for the aperture fields which is solved with the method of moments using rectangular waveguide modes as basis and test functions. An efficient uniform asymptotic technique is used to calculate the excitation vector and the backscattered far-field. The asymptotic solution is valid for large cylinders coated with thin dielectric layers away from the paraxial (i.e. near axial) region. A similar asymptotic solution is used to calculate the mutual coupling in the nonparaxial region. For the self coupling terms and for the mutual coupling in the paraxial region a planar approximation is used with a corresponding spectral domain technique. Numerical results are presented as a function of frequency, angle of incidence, cylinder radius, and electrical thickness of the coating.     

Heating patterns generated by phase modulation of a hexagonal arrayof interstitial antennas

In this paper, we investigate an array of six interstitial microwave antennas used for hyperthermia cancer treatment. The purpose is to generate both uniform and controlled nonuniform heating patterns in biological tissue by phase modulating the signals applied to each antenna. The array consists of six antennas positioned on the corners of a hexagon. The distance between two diagonal antennas is 4 cm. The distributions of absorbed power per unit mass within the array are computed, and then converted into temperature distributions through a thermal conduction simulation. The SAR and temperature patterns are presented in both the lateral plane (perpendicular to the antennas) and the axial plane (parallel with the antennas). By proper phase modulation of microwave signals applied to each antenna, a uniform heating pattern can be produced within the entire array volume. Also, a peripheral heating pattern may be generated around the array; again, by using the proper phase modulation. The modulation schemes for generating both types of heating patterns are discussed.     

Radiation and scattering tradeoff design for conformal arrays

A tradeoff between gain and radar cross section is presented for waveguide aperture antennas mounted on a circular cylinder. The radiation and scattering characteristics are investigated as a function of frequency, various array lattices and different impedance loads using a method of moments code combined with asymptotic techniques. First, single aperture elements in a cylinder are considered and then different arrays. It is shown that it is possible in some cases to maximize the radiation and minimize the scattering properties with a suitable choice of impedance loads. The importance of mutual coupling is investigated and it is found that under certain circumstances it is possible to draw conclusions from calculations where mutual coupling has been omitted. The radiation and scattering properties of cylindrical arrays have been compared to the corresponding quantities for planar arrays and the advantages/disadvantages of the two concepts are illustrated.     

Interleaved Ultrawideband Antenna Arrays Based on Optimized Polyfractal Tree Structures

There is considerable interest in interleaving multiple phased array antennas into a single common aperture system. Current phased array antenna technology is limited to narrowband operation, leading to the appearance of grating lobes and strong mutual coupling effects when they are incorporated into the design of a common aperture system. To overcome this obstacle, a new class of arrays, called polyfractal arrays, has been introduced that possess natural wideband properties well suited for large-scale genetic algorithm optimizations. These arrays also possess recursive beamforming properties and an autopolyploidy-based chromosome expansion that can dramatically accelerate the convergence of a genetic algorithm. In addition, a robust Pareto optimization can be applied to reduce the peak sidelobe levels at several frequencies throughout the intended operating band, leading to ultrawideband antenna array designs. Because of their lack of grating lobes, these polyfractal arrays are ideal building blocks for interleaved antenna array systems. This paper develops these concepts, first creating ultrawideband array designs based on polyfractal geometries and then interleaving these designs into a common aperture system. Several examples of interleaved systems are discussed, with one two-array system possessing a peak sidelobe level of nearly -18 dB with no grating lobes over a 20:1 bandwidth with either of the component array mainbeams steered independently up to 60deg from broadside.     

The development of inflatable array antennas

Inflatable array antennas are being developed to significantly reduce the mass, the launch vehicle stowage volume, and the cost of future spacecraft systems. Three inflatable array antennas, previously developed for spacecraft applications, are a 3.3 m×1.0 m L-band synthetic-aperture radar (SAR) array, a 1.0 m-diameter X-band telecom reflectarray, and a 3 m-diameter Ka-band telecom reflectarray. All three antennas are similar in construction, and each consists of an inflatable tubular frame that supports and tensions a multi-layer thin-membrane RF radiating surface with printed microstrip patches, The L-band SAR array achieved a bandwidth of 80 MHz, an aperture efficiency of 74%, and a total mass of 15 kg. The X-band reflectarray achieved an aperture efficiency of 37%, good radiation patterns, and a total mass of 1.2 kg (excluding the inflation system). The 3 m Ka-band reflectarray achieved a surface flatness of 0.1 mm RMS, good radiation patterns, and a total mass of 12.8 kg (excluding the inflation system). These antennas demonstrated that inflatable arrays are feasible across the microwave and millimeter-wave spectrum. Further developments of these antennas are deemed necessary, in particular, in the area of qualifying the inflatable structures for space-environment usage     

Coupling phenomena in concentric multi-applicator phased arrayhyperthermia systems

The coupling between the waveguide applicators of a four-element phased array hyperthermia system irradiating a three-layered cylindrical tissue model of circular cross section is analyzed theoretically. The fields inside the tissue layers are expressed in terms of cylindrical vector wave functions satisfying the corresponding wave equations, while the fields inside each waveguide are expanded in terms of guided and evanescent normal modes. Then, by implementing the appropriate boundary conditions, a system of four coupled integral equations is derived in terms of the unknown electric field distributions on the open waveguide apertures. This system is solved by expanding the unknown electric field on each aperture into waveguide normal modes and by applying a Galerkin's procedure. The self reflection coefficient and the mutual coupling coefficients are then determined and numerical results for a four-element phased array hyperthermia system are computed and presented for different waveguide applicator sizes and settings     

Mutual coupling in arrays of coaxial waveguides and horns

A solution is presented for the mutual coupling between the modes of different-sized circular coaxial aperture arrays, which are used as direct radiators and feed antennas. This solution is expressed in terms of integral transforms and the mutual admittances for the cross coupling between all three mode types, TE, TM, and TEM, are obtained. The accuracy of this new integral formulation is demonstrated by comparing computed results with measured data for a three-element test array. Excellent agreement is obtained between theory and experiment for both coupling coefficients and radiation patterns. Results are also presented for a coaxial feed array for the Lovell radio telescope.